In-mold decorating of resin containers with a label has conventionally been carried out by injection molding, blow molding, differential pressure forming, expansion molding, and the like with a label being previously inserted into a mold. Labels known for in-mold decorating include gravure-printed resin films, full color offset-printed synthetic paper (see, for example, JP-B-46-40794 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-B-54-31030, and British Patent 1090059), and a gravure-printed aluminum foil supported by a polyethylene film.
As long as the label or blank is integrally molded with a molten thermoplastic resin under high pressure, as in injection molding (100 to 1000 kg/cm.sup.2), the resulting labeled container has a satisfactory appearance. However, where molding is conducted under low pressure, as in differential pressure forming (2 to 7 kg/cm.sup.2) or blow molding (1 to 10 kg/cm.sup.2), air is not allowed to escape sufficiently from the space between the blank and the molten container, resulting in a poor appearance due to trapped air-pockets called blisters.
In an attempt to eliminate blisters in blow molding or differential pressure forming, use of multi-color printable synthetic paper having a multi-layer structure of stretched resin films as a label has been proposed as disclosed in U.S. Pat. No. 4,986,866 and EP-A-436044. The proposed synthetic paper label is obtained by preparing a multi-layer film composed of a biaxially stretched polypropylene base layer having on one side thereof a paper-like layer comprising a uniaxially stretched polypropylene film containing 8 to 65% by weight of an inorganic fine powder and on the other side thereof an adhesive layer comprising a heat-sealable resin film having a melting point of 85.degree. to 135.degree. C., embossing the heat-sealable resin film, and then stretching the multi-layer film having an embossed finish on the back side thereof at a temperature higher than the melting point of the heat-sealable resin and lower than the melting point of the inorganic fine powder-containing thermoplastic resin. The thus obtained label is fixed in a mold with the paper-like layer in contact with the inner wall of the mold, a molten polyethylene compound is then adhered onto the adhesive layer of the label under pressure or under reduced pressure, followed by cooling.
Since the label used comprises a propylene resin whose melting point is higher than that of polyethylene as a molding compound, the above-described process holds an advantage that the label hardly suffers from shrinkage marks despite being composed of stretched films.
In the production of hollow containers, improvement in productivity is generally achieved by increasing the mold cooling efficiency to shorten the molding cycle. Besides, molding is commonly carried out at a parison temperature set about 40.degree. to 80.degree. C. higher than the melting point of the molding resin, while varying depending on the shape or size of the container or the mode of molding (e.g., a rotary blow molding machine or a shuttle type blow molding machine). In these cases, it is likely that the heat-sealable resin film of the molded-in label will not have cooled sufficiently, so when the container is withdrawn from the mold its label is at a high temperature, resulting in frequent occurrences of blisters. If the resin of the heat-sealable film is kept at a temperature above its melting point at the time when the labeled container is taken out, the label fails to follow the after-mold shrinkage of the container and undergoes lifting from the container, which appears to be one of the great causes of blisters.